During early centuries the art of metal cutting made little progress except in-so-far as the application of greater driving power and the use of better machines were concerned. Lathes had been known since the sixth century B. C., at least, but, of course, little was or could be accomplished, comparatively speaking, before the invention of the steam engine by Watt, which was the first contrivance to give sufficient power for machining purposes. During the century which followed Huntsman’s time, steel makers and smiths became very expert in the manufacture and tempering of carbon steels. Lathe tools were made of these carbon steels, the only available material, but even with the better machine shop practice of the 19th century they were capable only of what we now consider to be inefficient results. The trouble was that since carbon tool steel gets its hardness from quenching from a red heat and then “drawing” the temper by reheating and slowly cooling from 400° or 500° F., tools made from it could not retain their hardness if their cutting points became much heated, as occurred if the lathe was run too fast. The usual cutting speed, therefore, was 20 or 30 linear feet per minute. Speeds in excess of this took the temper out of the tools and soon made them useless.
About 1868 Robert F. Mushet, a metallurgist of Sheffield, England, made a momentous discovery. He found that a piece of tool steel which had cooled in the air was as hard as some of those which he had quenched. Being an investigator he set about discovering the reason for this experience which was without precedent. Analysis showed that beside the usual constituents of tool steel this particular bar contained tungsten, a comparatively new metal. He experimented with some hundreds of mixtures and evolved an alloy, which, in tools, would stand up under machine speeds double those which could be used with carbon steels. These new alloys became known as “air-hardening” or self-hardening tool steels because they required no quenching.
The principal application of these new steels was in the cutting of harder metal than it had before been possible to cut, and little attention, apparently, was paid to the getting of greater outputs by increase of machine speeds.
At this point Frederick W. Taylor of “efficiency” fame appears upon the scene. While manager of the Bethlehem Steel Works in the nineties of the last century, he was working upon the efficiency investigations for which he later became so famous. During his investigations, with Maunsel White he experimented with many air-hardening steels to determine the best grades to use for their shop work. Getting some inconsistent results they determined upon and made what was a most extended and systematic investigation, one so thorough and complete that Taylor and White have become part of the history of high-speed steels.
They produced new compositions the quenching of which could be from temperatures greatly in excess of those which tool makers for centuries had held to be ruinous and which really are ruinous to carbon tool steels. The best results were obtained when the new steels were quenched by plunging in oil from close to their melting points—a dripping or “sweating” heat as it is called. This was something entirely new but it developed that after proper drawing, steels quenched from these extreme temperatures, would stand up under lathe speeds as great as 200 or 300 feet per minute.
Compare these with the miserable speeds of 20 or 30 feet per minute, which were the average performances of the best carbon steels.
The secret of Taylor and White’s treatment was not long in coming out and soon high-speed steel makers in Europe and America were vying with each other in production of finer and finer high-speed steels.
The progress which has been made during the last twenty years and particularly during the past ten has been astounding. Improvement has followed improvement in composition, manufacture and heat treatment, so that to-day, instead of the cut at 30 feet per minute, which was a high figure with carbon steel tools, the modern lathe or shaper tool often works at 300 or 400 or more feet per minute, and, with sufficient power behind it, at somewhat lower speeds plows out ½ inch deep and ¾ inch wide chips so fast that their removal to keep the machine clear is no mean problem. Often 2,000 pounds of the material per hour can be thus cut away with one tool.
As suggested, the new steels do not suffer such loss of temper from the heat generated by the friction of the tool in the metal as occurs with carbon steel tools. In fact, tools of high-speed steel work best after “warming up” and they can run for a considerable period of time with the point of the tool red-hot, though such is not advisable.
As is readily seen the essential property of the high-speed steels is the so-called “red hardness” which is the ability to retain hardness at red heat. This is several hundred degrees in excess of the temperatures at which the carbon tool steels quickly lose their “temper.”